1. Field of the Invention
The invention is related to the field of two-wire bus instruments, and in particular, to an entirely intrinsically safe two-wire bus instrument for a hazardous environment.
2. Statement of the Problem
Flowmeters are used to measure the mass flow rate, density, and other characteristics of flowing materials. The flowing materials can comprise liquids, gases, combined liquids and gases, solids suspended in liquids, and liquids including gases and suspended solids. For example, flowmeters are widely used in the well production and refining of petroleum and petroleum products. A flowmeter can be used to determine well production by measuring a flow rate (i.e., by measuring a mass flow through the flowmeter), and can even be used to determine the relative proportions of the gas and liquid components of a flow.
One type of flowmeter environment is a hazardous environment where combustible vapors or particles are present. An instrument operating in such conditions must be designed and manufactured to avoid igniting the combustible vapors or particles. An instrument designed to safely operate in a hazardous environment is typically designated as being “intrinsically safe” (I.S.). For example, such an instrument typically uses low level electrical voltages and low level electrical currents that are known to be unable to cause ignition in a hazardous environment.
FIG. 1 shows a two-wire bus according to the prior art, such as a FIELDBUS™ two-wire bus commonly used for industrial instrumentation. The term FIELDBUS™ refers to a two-wire instrumentation bus standard that is typically used to connect together multiple instruments and is further capable of being used to provide digital communications between instruments. In addition, the fieldbus barrier device can pass digital communication signals to external devices, such as to monitoring and control stations, for example. The bus barrier device is connected to an electrical power source and provides electrical power over the two-wire bus. The bus barrier device typically receives non-I.S. electrical power and in turn provides electrical power that is current limited, voltage limited, and power limited.
Several FIELDBUS™ standards exist. One FIELDBUS™ standard specifies that the barrier device can provide a maximum current of about 130 milliamps (mA) and a maximum voltage of about 15 volts (V) to all connected instruments. In the prior art, typical fieldbus instruments were designed to draw about 10 to 20 mA. Therefore, in a prior art approach, the number of devices that can be connected to the two-wire bus is controlled by the total current consumption of the devices at the available barrier voltage.
FIG. 2 shows a typical prior art instrument connected to a two-wire bus, showing how electrical power from the two-wire bus is used. The prior art instrument includes a signal processor and an electrical interface. The electrical interface connects to a two-wire bus, such as a FIELDBUS™, and to a power source. The electrical interface includes a bus electrical barrier that connects to the two-wire bus. The bus electrical barrier provides electrical isolation from the bus and provides current, voltage, and power limiting of the electrical power taken from the two-wire bus. The two-wire bus provides Direct Current (DC) electrical power to the bus electrical barrier. The electrical interface further includes a signal electrical barrier that connects to the signal processor. The signal processor is connected to a sensor, as shown. The signal electrical barrier provides electrical isolation from the signal processor and provides current, voltage, and power limiting of the electrical power provided to the sensor via the signal processor. Where the sensor is a flowmeter sensor, such as a Coriolis flowmeter, the signal processor is typically connected to the sensor by a nine-wire cable. The electrical interface further includes a communication system. The communication system receives a data signal from the signal processor and modulates the data signal onto the two-wire bus as a digital communication signal. The FIELDBUS™ standard dictates that digital communication signals occur in a frequency band centered around 32 kilohertz (kHz).
It should be understood that in this prior art configuration, the electrical power taken from the two-wire bus is used only to power the communication system. A minimal amount of electrical power is therefore taken from the two-wire bus. For example, the communication system typically draws only about 10-20 mA from the two-wire bus. The external power source in the prior art provides the relatively large electrical power that is used to operate drivers or active elements (such as a flowtube driver(s)) and that is used to power the signal processor. As a result, making the instrument intrinsically safe is difficult and complex. Consequently, the electrical interface must include the bus electrical barrier and the signal electrical barrier. Furthermore, the instrument itself is often enclosed in an explosion-proof housing. This is done in the prior art because the power source is not intrinsically safe, even though the FIELDBUS™ two-wire bus standard originated as an intrinsically safe bus system.